“Experimental studies on microstructural and corrosion characterisation of additively manufactured AISI 316 L austenitic stainless steel”

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Experimental studies on intergranular and pitting corrosion characterisation of additively manufactured AISI 316 L austenitic stainless steel

CERTIFICATE

The undersigned certify that they have read and recommended to the Department of Mechanical Engineering for acceptance, a project report entitled

“Experimental studies on intergranular and pitting corrosion characterisation of additively manufactured AISI 316 L austenitic stainless steel”

Dr. Amandeep Singh Shahi
Professor & Head (Department of Mechanical Engineering)
Sant Longowal Institute of Engineering & Technology, Longowal

ACKNOWLEDGEMENT

It gives us immense pleasure to express our deepest sense of gratitude and sincere thanks to our highly respected and esteemed guide Dr. A.S Shahi, for his valuable guidance, encouragement and help for completing this work. His useful suggestions for this whole work and co-operative behaviour are sincerely acknowledged.

We would like to express our sincere thanks to Dr. J.S Gill (Course Counsellor), for giving us this opportunity to undertake this project. We would also like to thank. Dr. A. S Shahi (Head of the department) for whole hearted support.

We are also grateful to the support offered by Welding metallurgy Laboratory, SLIET towards completion of this project work.

At the end we would like to express our sincere thanks to all our friends and others who helped us directly or indirectly during this project work.

ABSTRACT

Microstructure and pitting corrosion behaviour of additively manufactured wall of AISI 316L was investigated. The results indicated that weld zone, including ferrite and austenite phases, was mainly composed of columnar dendrites. No obvious re-passivation was shown in as-welded and thermally aged samples. Pitting occurred in both the as-weld and heat treated samples. However, the pitting potential was less in thermally aged sample signifying less pitting resistance.

Keywords: Wire-Arc Additive Manufacturing; Corrosion, Microstructure, Stainless steel

Chapter 1: Introduction

Austenitic stainless steel is widely used as the structural material in many fields for its outstanding mechanical and corrosion properties. [1] Welding is extensively selected to connect these structural materials, and then the microstructure variation exists in the weld joint.[2] It is believed that the microstructure discrepancy results in different local corrosion behaviour such as pitting corrosion and intergranular corrosion and stress corrosion cracking during service.[3]

AISI 316 L austenitic stainless steel is used as implant materials to make such devices as artificial joints, bone plates, stents and so on, thanks to its favourable combination of mechanical properties, corrosion resistance, satisfactory biocompatibility and relatively low cost compared with other metallic biomaterials [4].

However, after its implantation in human bodies, sometimes mechanical failure occurs and tissue inflammation may happen, which makes safety become dissatisfactory and insufficient for its biomedical applications [5]. Because of the high concentration of Cl− and the temperature range of 36.7–37.2 °C, the human body fluid is considered a severely corrosive environment and localized corrosions such as pitting, crevice corrosion and fretting fatigue are probable to appear on AISI 316L steel [6].

Metal ions such as iron, chromium and especially nickel could be released and thus cause toxicity to the body and deteriorate the AISI 316L biocompatibility [7]. Therefore, improvement of the biocompatibility of AISI 316L steel, especially its blood compatibility when used in the vascular environment, can be beneficial to its safe use in the human body.

The objective of this work is to investigate the microstructure and pitting corrosion behaviour of AISI 316L stainless steel additively manufactured using WAAM.

Chapter 2: Background and Literature Review

2.1 Literature review

According to the previous works, the weld joint is mainly composed of weld zone, heat-affected zone (HAZ), and base metal. Normally, it is accepted that weld zone possesses a duplex structure including ferrite and austenite phases and the ferrite in the weld zone reduces the hot cracking susceptible of the weld joint, while Kwok [8] also found that the presence of ferrite in laser-welded SS304 stainless steel joint contributed to a decrease in pitting corrosion resistance. The HAZ, caused by thermal cycle during welding process, exhibits to be more susceptible to pitting corrosion compared with base metal. Second phase precipitation, recrystallization, and residual stress are proposed as the accelerating factors for pitting corrosion resistance, degradation of HAZ and it is reported that HAZ is the most sensitive zone for pitting corrosion. However, Lu [9] found that weld zone had a lower breakdown pitting potential than HAZ under gas tungsten arc welding process, confirming the existence of pitting corrosion performance discrepancy of weld zone under different welding methods and conditions.

Stainless steel is an alloy of iron and several other elements (such as nickel, chromium, molybdenum, and carbon) and due to these elements it is more resistant to corrosion than plain iron or steel (which is simply iron and carbon).These stainless steel elements, such as nickel, chromium, and other additives, give it a passive oxide layer that resists the formation of rust and creates a shiny, reflective surface. This passive layer has the unique ability to repair itself. The shiny surface of stainless steel is very difficult to tarnish compared to plain steel, hence why it is called “stainless” steel. It is considered environmentally neutral and inert that is why it has longevity because it does not leach compounds that could modify its composition when in contact with elements like water and air. The four grades of stainless steel have been classified according to their material properties and welding requirements:

Austenitic stainless steels has a faced-centered cubic (FCC) crystal structure and a grain structure consisting of austenite. It has a Cr-level of 16 - 25 % and an addition of up to 35 % Ni to stabilize the austenitic structure. Since so much Ni is added it also makes the austenitic stainless steels more expensive than ferritic stainless steels. They are non-magnetic and have a good formability and weldability. The temperature use-span is wide, from temperature below 123 K up to red-hot temperatures. Austenitic stainless steels has a wide use and it often used in aircraft applications, food and dairy industry and pulp and paper manufacturing.

Ferritic stainless steels has a body-centred cubic (BCC) crystal structure and a grain structure consisting of ferrite. The Cr-level spans from 10.5 % to greater than 25 %. Their low cost made it grow in use and they are well suited for use as light-gauge sheets. They are ferromagnetic, contains a low amount of carbon and are for some alloys poor for welding in thicker-walled pieces due to the formation of brittle martensite. They cannot be hardened by heat treatment, but strengthen by annealing. Ferritic stainless steels has a wide use in automotive exhaust systems and kitchen applications

Martensitic stainless steels have either a martensitic α ′ - or ǫ-phase. Martensite forms at high cooling rates, when the transformation of austenite happens diffusionless. They have a low corrosion resistance due to the low amount of alloying elements used to keep the martensitic phase stable. This reduces the corrosion resistance, but they still fill a use since they have very good mechanical properties such as high strength and hardness. They are perfect for high-temperature applications where a strong and stable material is needed.

Duplex stainless steel is the newest type of stainless steel alloy. It is a combination of austenitic and ferritic structure to achieve a high strength. They have a high corrosion resistance due to the amount of Cr being more than 20 %. They are less expensive than austenitic stainless steels due to the low amount of Ni used. They are often used in -100 – 300 ◦C applications where austenitic stainless steels have been used before and where high strength is needed.

Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical. The microstructure of a materials can strongly influence physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature behaviour or wear resistance. These properties in turn govern the application of these materials in industrial practice.

Steel properties come from their microstructural phases, small gaps between atoms, called interstices, are where small elements like carbon and nitrogen fit. As the alloying increases, the straining in the atomic lattice increases, requiring more force to deform the work piece, thereby increasing the strength.

When a very small fraction of the interstices in between the iron lattice is occupied by carbon atoms, this interstitial-free steel is said to have a microstructure of ferrite. Ferrite has a body-centered cubic (BCC) crystal structure. Ferrite is a microstructural phase that is soft, ductile, and similar to pure iron. There is a limit on how much carbon can fit in the gaps in the ferrite structure: 0.02 percent carbon at 1,340 degrees F (725 degrees C), but dropping to 0.006 percent (60 PPM) carbon at room temperature.

The gaps are a little larger in a phase known as austenite, which has a face-centered cubic (FCC) crystal structure. At around 2,100 degrees F (1,150 degrees C), up to 2 percent carbon can fit into the austenite microstructure.

As the steel slowly cools from this temperature and carbon is forced out of solution, the austenite transforms into a combination of ferrite and another phase called cementite, also known as iron carbide, which has the chemical composition of Fe₃C. The amount of cementite that forms is a function of how much carbon is in the steel. Because ferrite cannot contain more than about 60 PPM carbon at room temperature, the rest of the carbon winds up as cementite.

Unlike ferrite, cementite has the characteristics of a ceramic: very hard and brittle, with low toughness and little resistance to crack initiation and propagation. The mixture of ferrite and cementite is called pearlite, named because it looks like mother of pearl under a microscope, with alternating layers of ferrite and cementite.

With faster cooling, different dynamics occur. Above a critical cooling, the excess carbon of the FCC austenite does not have time to diffuse out of the crystal structure and form cementite. Instead, the carbon is trapped in with the now nearly pure iron and forced into the interstitial locations that are not large enough to accommodate the carbon atoms. This distorts and strains the crystal matrix into a body-centered tetragonal (BCT) structure, forming a hard phase called martensite.

At higher carbon levels, more carbon is frozen into the BCT structure, further straining the crystal matrix. This is why the hardness of martensite increases with carbon level. The volume of the BCT martensite structure is larger than that of the FCC austenite, so the freshly transformed martensite is compressed by the surrounding matrix.

If martensite is heated, carbon has the opportunity to diffuse out from the BCT structure, reducing the distortion of the crystal matrix, leading to decreased hardness and increased toughness. This heat treatment produces a microstructure of ferrite and iron carbide (Fe₃C) called tempered martensite.

Bainite is another microstructure that can form when austenite is cooled. It typically consists of a combination of ferrite, cementite, and retained austenite. Because the cooling rate to form bainite is slower than the cooling rate needed to form martensite, carbon has some opportunity to diffuse out of the FCC austenite, allowing for the formation of BCC ferrite. Bainitic microstructures have the best balance of strength and ductility. The cooling rate is fast enough to increase the strength, while the rounded hard microstructural constituents are not as prone to crack initiation and propagation as if they were flat and elongated.

Figure 2-Schematic diagram showing time-temperature transformation curve for plain carbon eutectoid steel [10]

During the solidification of a pure metal the solid-liquid interface is usually planar, unless severe thermal undercooling is imposed. During the solidification of an alloy, however, the solid-liquid interface and hence the mode of solidification can be planar, cellular, or dendritic depending on the solidification condition and the material system involved.

Figure 3-Basic solidification modes: (a) planar solidification, (b) cellular solidification, (c) columnar dendritic solidification, (d) equiaxed dendritic solidification [11]

Typical microstructures resulting from the cellular, columnar dendritic, and equiaxed dendritic modes of solidification in alloys are shown below-

Figure 4-Nonplanar solidification structure in alloys. (a) Transverse section of a cellularly solidified Pb–Sn alloy, (b) Columnar dendrites in a Ni alloy, (c) Equiaxed dendrites of a Mg–Zn alloy, (d) Three-dimensional view of dendrites in a Ni-base super-alloy [11]

Chapter 3: Corrosion in materials

Galvanic corrosion is also known as bimetallic corrosion, as the name suggests two different types of metal, when two dissimilar metals kept together directly or indirectly, in one of the metals corrosion rate occurs at a faster pace and the other metal remains unaffected. There is basically a galvanic couple that forms between the metals and one become anodic and the other cathodic. The main factor which plays the role is the magnitude of the potential difference between the metals.

Generally above 500ºC high-temperature corrosion takes place. Although there are many things that occur at high temperatures one of the main things is oxidation. Because of this the loss in material changes in mechanical properties because of the change in the microstructure of the alloy, and many more occurs.

In this corrosion, grain boundaries are under attack, and carbides are formed, further propagating for higher corrosion. The cause of this corrosion, in general, is improper heat treatment. In austenitic stainless steel, chromium carbides can become precipitated at the grain boundaries which reduces the individual chromium concentration and makes boundaries prone to corrosion.

The most common form of localized corrosion is pitting corrosion. As the name suggests it forms a hole of a small diameter that penetrates into the surface, generally visible to naked eyes and the remaining surface is intact. The leading causes of pitting corrosion are lack of passive film, high aggressive medium in which the metal exists, poor coating application, and much more.

One of the highly penetrating types of localized corrosion is a crevice, which occurs due to a small gap in the surface of the metal for example cracks, seams, small spaces that occur during manufacturing, and much more.

Corrosion that occurs under the coated surface on the main metal. Occurs due to the defects in the coating of the surface when some external elements attack the main metal because of the defect in the protective layer.

The corrosion occurs due to the tensile, bending, and torsion stress generally often at rapid high temperatures. The only prevention for SCC is to choose proper fabrication with suitable material and suitable thermal and mechanical stresses.

3.2 Electrochemical techniques

The condition of corrosion in a concrete or metal element is determined through electrochemical corrosion testing. Electrochemical corrosion testing, which is based on electrochemical theory, evaluates corrosion damage and, when possible, determines corrosion rates. All corrosion is an electrochemical reaction involving oxidation and reduction. To assess the corrosion characteristics of metals and metal components in conjunction with various electrolyte solutions or soil conditions, controlled electrochemical experimental approaches can be utilised. Each metal/solution system has its own corrosion properties. The ASTM/NACE methods, Cyclic Polarization, Impedance Spectroscopy, linear polarisation tests, electrochemical testing for hydrogen permeation, real-time corrosion forecasts for materials, and others are among the electrochemical techniques.

3.2.1 Cyclic Potentiodynamic Polarization (CPP)

CPP test was introduced for the first time in the 1960s. CPP measurements should be carried out according to the defined ASTM standards (F2129, G61). It is widely used to determine resistance to localized corrosion or degradation rate in a short time. Thus this technique is applicable as a method for prediction of localized corrosion also beneficial for alloys that are passivized spontaneously and underwent localized corrosion.

General shape of CPP curve is as follows; after passing through the region of active corrosion, the current density decreases to a critical potential, called the “Flade potential” or “primary passivation potential”. This decrease is due to the formation of the passive layer on metal surface.

The passive current density is the current density in the passive region. With further increase in the potential in the passive region, a rapid rise in the anodic current can be detected. This rise is due to either the evolution of oxygen by the decomposition of water or breaking the passive film and localized corrosion. If the increase of current density is due to the decomposition of water and evolution of oxygen gas, the region is called “transpassive region”

3.3 Corrosion control techniques

Selection of the right material of construction, Surface coating, use of Inhibitors, proper equipment design and electrical protection

One of the easiest and cheapest ways to prevent corrosion is to use barrier coatings like paint, plastic, or powder. Powders, including epoxy, nylon, and urethane, adhere to the metal surface to create a thin film. Plastic and waxes are often sprayed onto metal surfaces. Paint acts as a coating to protect the metal surface from the electrochemical charge that comes from corrosive compounds. Today’s paint systems are a combination of different paint layers that serve different functions. The primer coat acts as an inhibitor, the intermediate coat adds to the paint’s overall thickness, and the finish coat provides resistance to environmental factors.

The biggest drawback with coatings is that they often need to be stripped and reapplied. Coatings that aren’t applied properly can quickly fail and lead to increased levels of corrosion. Coatings contain volatile organic compounds, which make them hazardous to people and the environment.

This corrosion prevention method involves dipping steel into molten zinc. The iron in the steel reacts with the zinc to create a tightly bonded alloy coating which serves as protection. The process has been around for more than 250 years and has been used for corrosion protection of things like artistic sculptures and playground equipment.

Unfortunately, galvanization can’t be done on-site, meaning companies must pull equipment out of work to be treated. Some equipment may simply be too large for the process, forcing companies to abandon the idea altogether. In addition, zinc can chip or peel. And high exposure to environmental elements can speed up the process of zinc wear, leading to increased maintenance. Lastly, the zinc fumes that release from the galvanizing process are highly toxic.

Alloyed steel is one of the most effective corrosion prevention methods around, combining the properties of various metals to provide added strength and resistance to the resulting product. Corrosion-resistant nickel, for example, combined with oxidation-resistant chromium results in an alloy that can be used in oxidized and reduced chemical environments. Different alloys provide resistance to different conditions, giving companies greater flexibility.

Cathodic protection protects by electrochemical means. To prevent corrosion, the active sites on the metal surface are converted to passive sites by providing electrons from another source, typically with galvanic anodes attached on or near the surface. Metals used for anodes include aluminum, magnesium, or zinc.

While cathodic protection is highly effective, anodes get used up and need to be checked and/or replaced often which can drive up costs of maintenance. They also increase the weight of the attached structure and aren’t always effective in high-resistivity environments.

Chapter 4: Methodology and experimental details

4.1 Methodology

4.2 Experimental details

The experimental details of the microstructure evaluation and corrosion examination of WAAM manufactured AISI 316L welds is given as follows-

For carrying out the microstructural evaluation of the fabricated welds, samples were cut from the fabricated walls and the surfaces were sanded and polished down up to 3000 grit of emery paper .These samples were etched using carpenter’s etchant. Each sample was immersed in the etchant at 20^oC and then water rinsed.

Carpenters Stainless Steel Etch	FeCl₃	8.5 grams	Immersion etching at 20 degrees Celsius	For etching duplex and 300 series stainless steels
	CuCl₂	2.4 grams
	Hydrochloric acid	122 ml
	Nitric acid	6 ml
	Ethanol	122 ml

An inverted type optical metallurgical microscope was used to examine microstructure on etched samples.

Corrosion characterisation of the welds was done using a potentiostat (Gamry Instruments, model: Reference 600) supported by Gamry framework software. For conducting cyclic potentiodynamic polarization (CPP) test in a paracell at room temperature, where three electrodes, namely a saturated calomel electrode (SCE) as a reference electrode, graphite counter electrode and working electrode (weld test sample), were used. Each sample was exposed to an electrolytic solution of 0.5 M H₂SO₄ (sulfuric acid) + 0.5 M NaCl (sodium chloride).

Chapter 5: Results and Discussions

Weld metal microstructures of the test samples are presented in Figures 6(a) through (b). The microstructure of the HAZ is characterised with dendrites. The weld zone is solidified to form austenite + ferrite matrix in additively manufactured AISI 316L. The figure 6(a) represents individual fusion boundaries of additively manufactured layers. Each HAZ and fusion zone is clearly visible in the microstructure. The grains are closely packed in the HAZ region and are loosely spaced in the weld zone.

Figures 7(a) to 7(c) shows the microstructure of WAAM wall taken at different positions. The figures reveal that in all the three locations austenite matrix was present throughout the additively manufactured wall

Figure 8- Image showing microstructures of additively manufactured WAAM wall (a) At the bottom-layer, (b) At the middle layer, (c) At the top layer

Pitting potential - The least positive current and voltage at which pits develop or grow on a metallic surface. This is the electrochemical potential in a given environment above which a corrosion pit initiates on a metallic surface

Corrosion potential - Corrosion potential is a mixed potential (also an open-circuit potential or rest potential) at which the rate of anodic dissolution of the electrode equals the rate of cathodic reactions and there is no net current flowing in or out of the electrode.

Repassivation Potential - Pits repassivate below the repassivation potential, denoted by E_rp

Initial E (V)	-0.25
Apex E (V)	1.5
Final E (V)	0
Forward Scan (mV/s)	1
Reverse Scan (mV/s)	1
Sample Period (s)	1
Apex I (mA/cm^2)	10
Density (g/cm^3)	7.805
Init. Delay Time (s)	300

Figure 10- Weld sample after CPP test (significant pits observed on the surface)

Initial E (V)	-0.25
Apex E (V)	1.5
Final E (V)	0
Forward Scan (mV/s)	1
Reverse Scan (mV/s)	1
Sample Period (s)	1
Apex I (mA/cm^2)	10
Density (g/cm^3)	7.805
Init. Delay Time (s)	300

Figure 11-Cyclic potentiodynamic polarisation test results for thermally-aged sample

Figure 12-Thermally aged weld sample after CPP test (significant pits observed on the surface)

Figure 14-Image showing sample preparation (a) WAAM wall manufactured for experimentation, (b) Samples cut for examination, (c) Polished sample

Figure 15- Equipment used in the present work (a) Muffle furnace used for thermally ageing the samples, (b) Potentiostat used for conducting corrosion tests

Chapter 6: Conclusions

The microstructure and pitting behaviour of AISI 316L stainless steel WAAM wall was investigated. Microstructures in the additively manufactured wall was examined by OM, the pitting corrosion of the as welded and heat treated samples of the stainless steel wall was investigated by potentiodynamic polarization tests using the neutral chloride electrolyte 17.5 gm NaCl in 500 mL solution. The results were obtained as follows:

References

[1] J.L. Cardoso and R.C.A. Vieira: Pitting corrosion resistance of austenitic and superaustenitic stainless steels in aqueous medium of NaCl and H2SO4. J. Mater. Res. 31, 1755 (2016).

[2] Y. Cui, C.D. Lundin, and V. Hariharan: Mechanical behavior of austenitic stainless steel weld metals with microfissures. J. Mater. Process. Technol. 171, 150 (2006).

[3] K. Nishimoto and K. Ogawa: Corrosion properties in weldments of stainless steels (1). Metallurgical factors affecting corrosion properties. Weld. Int. 13, 845 (2010)

[5] E. Leitao, R.A. Silva, M.A. Barbosa, J. Mater. Sci., Mater. Med. 8 (1997) 365.

[7] R. Köster, D. Vieluf, M. Kiehn, M. Sommerauer, J. Kähler, S. Baldus, T. Meinertz, C.W. Hamm, Lancet 356 (2000) 1895

[8] C.T. Kwok, S.L. Fong, F.T. Cheng, and H.C. Man: Pitting and galvanic corrosion behavior of laser-welded stainless steels. J. Mater. Process. Technol. 176, 168 (2006).

[9] B.T. Lu, Z.K. Chen, J.L. Luo, B.M. Patchett, and Z.H. Xu: Pitting and stress corrosion cracking behavior in welded austenitic stainless steel. Electrochim. Acta 50, 1391 (2005)

[10] Schaeffler, D. J. (2016, August 1). A review of steel microstructures. The Fabricators.

[12] Zhu, P., Cao, X., Wang, W., Zhao, J., Lu, Y., & Shoji, T. (2017). An investigation on microstructure and pitting corrosion behavior of 316l Stainless Steel Weld Joint. Journal of Materials Research, 32(20), 3904–3911.

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